Reflux esophagitis is a common clinical disorder with the potential to lead to serious complications such as stricture and Barrett's (columnar-lined) esophagus, the latter a premalignant condition. Since contact of the esophageal epithelium with gastric acid is primarily responsible for the injury, therapy has been directed toward prevention of reflux, neutralization of acid and inhibition of acid secretion. These therapies however have had only limited success so that esophagitis continues to be a significant clinical problem. An alternative approach to the treatment of this disorder is by augmentation of epithelial defense mechanisms against acid (H+), and agents which enhance these mechanisms are designated as "cytoprotective." The specific aims of this proposal are: 1) to identify agents with cytoprotective properties against H+ injury to esophageal epithelium and 2) once identified, to explore the mechanisms behind their actions. Based on our prior research elucidating the mechanisms by which H+ injures the epithelium, two in vitro methods using the Ussing chamber have been developed for identifying agents with cytoprotective properties. Agents identified to have cytoprotective properties in vitro will be re-evaluated in vivo to confirm by morphological and functional (transport) criteria their protective effects against injury in the acid-perfused rabbit esophagus. Confirmation in vivo will be followed by investigations to define their mechanism(s) of action. As a starting point, the existing mechanisms for protection against H+-injury that are present in healthy esophageal epithelia will be investigated in vitro using Ussing chamber transport studies, electron microscopy with tracers, and pH microelectrode techniques. These investigations will seek to establish for rabbit and human esophagi: a) the locations and nature of the permeability barriers, b) the route(s) of H+ entry into the epithelium, c) the intracellular mechanisms for H+ removal, d) the rate of epithelial reconstitution after injury and e) the existence of a pH gradient across the mucus-unstirred water layer. Subsequently cytoprotective agents will be tested to determine if their protective effects against H+ injury are mediated through one or more of the established mechanisms. These studies ultimately may provide a fresh approach to the development of agents for the prevention and treatment of reflux esophagitis.